This paper presents an analytical and experimental research programme’s results regarding material nonlinearity’s influence on reinforced concrete beams’ immediate deflections. Six full-size laboratory beams were tested under different type of loads in the experimental programme. The ANSYS finite element software nonlinear material model was used for simulating the beams’ analytical behavior. Comparing ANSYS nonlinear material model results, ACI 318 equations and the results of the tests made revealed that a beam’s structural behavior is strongly influenced by material nonlinearity.